T passengers at Kendall Square in Cambridge, as at many area subway station, are on opposite platforms depending whether they’re headed inbound or outbound. They often have to pay a second time if they enter the wrong platform. (Photo: Marc Levy)

Design of the seven T stations to be built as part of the green line extension project doesn’t formally begin until fall, but there is one part of the plans that is certain: The stations will have central platforms on a single level, ensuring riders don’t have to leave and pay again if they realize they were headed in the wrong direction.

There are several stations where confused or distracted riders — including tourists or recent arrivals from other countries, groups Boston and Cambridge attract in large numbers — could find themselves having to pay a second time to correct going the wrong way, with Central and Kendall squares being prominent examples in Cambridge.

“All those stations were built a century ago. Certainly all the stations that have opened since World War II include that feature,” said Scott Hamwey, a planner with the Massachusetts Department of Transportation, referring to the ability to switch directions without paying again. “Although that would not be true if you were disabled.”

There is design work being done to address the problem with reconstruction at stations such as Orient Heights on the blue line, but other kinds of solutions would be necessary at stations such as Copley Square on the green line, which isn’t deep enough to have a mezzanine level, Hamwey said.

The green line extension project moves the Lechmere stop to the NorthPoint development side of Monsignor O’Brien Highway; adds a one-stop spur to Union Square in Somerville, roughly where Prospect Street and Webster Avenue meet; and lengthens the line through Somerville into Medford, including to Brickbottom, Gilman Square, Lowell Street, Ball Square, College Avenue and finally to Route 16, to a site near the Starbucks, Whole Foods Market and U-Haul depot. Officially, all but the outermost stop are to be built by October 2015.

Open to ideas

At Central, riders who pay to enter the platform for a train headed the wrong direction can be given a free ticket to get in the other side. The ticket is only good for a few minutes — just long enough to cross Massachusetts Avenue and get down to the opposite platform. But staffing levels at the station often leave one of the two platforms without an attendant who can issue a short-term ticket.

Hamwey said the department was open to hearing ideas for a technological solutions; he got a pitch for machines, one on each platform well inside the gates accepting Charlie Cards and passes, that would issue the short-term tickets. Similar outside solutions have been successful, he said, using as an example smartphone “apps” made by private developers that show how close a bus or subway car is to arriving at a station.

“We’re looking to expand that. It’s a pretty exciting development,” he said. “You have a way not to enter the subway station until you know a train is near.”

The innovation is worthless to people who don’t have smartphones or that app, he acknowledged, but there was potential for a grant proposal on the Fairmount commuter rail line (going from South Station to the southwest, including through Franklin Park, Dorchester, Mattapan and Hyde Park) that could bring train arrival announcements to LED signs outside stations.

Further questions on staffing, technology and design priorities at T stations were deferred by Hamwey to the department’s Joshua Robin, but messages left this week for Robin weren’t returned.